1. Field of the Invention
The present invention relates to the field of portable electronic devices that contain display devices. More particularly, the present invention provides an input sensing system which can detect an input in proximity to but not in contact with the surface of the display of an electronic device.
2. Prior Art
Advances in computer technology have enabled the further miniaturization of the components required to build computer systems. As such, new categories of computer systems have been created. One of the newer categories of computer systems is the portable, hand held, or “palmtop” computer system, also referred to as a personal digital assistant or PDA. Other examples of a portable computer system include electronic address books, electronic day planners, electronic schedulers and the like.
A palmtop computer system is a computer that is small enough to be held in the user's hand and as such is “palm-sized.” As a result, palmtops are readily carried about in the user's briefcase, purse, and in some instances, in the user's pocket. By virtue of its size, the palmtop computer, being inherently lightweight, is therefore exceptionally portable and convenient.
Flat panel resistive touch screen displays are found in numerous electronic products such as wrist watches, hand calculators, cell phones and PDAs both to present information to the user as well as to facilitate input of data such as user touch screen commands. Such displays include a resistive digitizer mechanism and a display mechanism. A typical resistive digitizer mechanism consists of a digitizing element having a flexible thin film supported slightly above the surface of another thin layer digitizing element. A pressure applied to the outer surface of the flexible film causes the film to deflect and contact the digitizing element at a point which can be measured and thereby used as an input signal to activate the digitizer mechanism.
The flexible film and the digitizing element must be mounted in a support housing to provide and maintain the proper spacing between the two. At the same time, additional protection against moisture, dust, and mechanical damage must be provided for the flexible film used in the digitizer mechanism. Thus, an additional outer protective film mounted above the digitizer flexible film is generally included in the touch screen display assembly. One problem with this technology is a gradual reduction in the sensitivity to the external mechanical pressure required to activate the digitizer mechanism. In addition, the added protective film may increase overall opacity which makes it more difficult to view any display element housed within.
FIG. 1 is a cross-sectional view of an enclosure/touch screen assembly 100 utilized in prior art. The entire assembly is held in place by the supporting structure 105. The outer protective film 110 provides mechanical protection for the resistive digitizer film 120. In addition, the outer protective film is coupled to the support mechanism in order to provide a moisture and dust seal. The digitizing element 130 is located below and close to the digitizer film 120. An externally applied pressure that deflects the protective film will also deflect the digitizer film.
Any applied pressure great enough to cause the digitizer film to contact the digitizing element will then activate the resistive digitizer mechanism. The display element 140 is located below the digitizer mechanism. User information is displayed on the upper surface of the display element. An icon sheet (not shown) is frequently disposed above display element 140 which delineates areas on the display screen for specific functions (e.g., button functions or a handwriting recognition area). Together, the protective film, the digitizer film and the digitizing element should have an opacity small enough to allow viewing of the information displayed on the display element.
The entire touch screen assembly is located within the support structure such that the surface of the outer protective film is below the upper edge of the support structure. There is therefore a step-down corner 150 from the upper edge of the support structure to the surface of the outer protective film and the resulting assembly exhibits a bezel like appearance. The support structure is also used to conceal the electrodes 160, insulators 170, and traces 180 which locate where the digitizer film and the digitizing element come into contact with each other.
One problem with the bezel design is that maintaining a moisture and dust-free environment for the touch screen mechanism is difficult. Such an assembly often does not provide a satisfactory moisture and dust proof enclosure. The bezel design adds to the complexity, cost of assembly, and overall thickness of the device. Additionally, some designers would like to eliminate the bezel to update the appearance of the device.
Another disadvantage with prior art display interfaces is the requirement that the digitizer assembly be a flat, two-dimensional surface. Currently, Indium Tin Oxide (ITO) is used as the conductive material (e.g., digitizer film 120 and digitizing element 130 of FIG. 1) in digitizer mechanisms. The digitizer mechanisms are formed by sputtering ITO onto a flat surface. The flat surface is required because while ITO has some flexibility, it is very brittle and will break down over time. Because of this ITO degradation, the interface of hand held computers needs to be re-calibrated occasionally. The brittleness factor necessitates an ITO layer that is flat and of uniform thickness. Additionally, the junction where the flexible digitizer film joins the electrodes and traces is frequently prone to failure.
The brittleness of the ITO limits the design of prior art devices to a flat interface which may not be suitable for some designs and makes a three-dimensional display surface impractical. A three-dimensional display would allow designers to contour the top surface such that it is not flat giving the device, for example, a curved top profile. The brittleness of the ITO also prevents using a printed decorative border sheet to conceal the electrodes and traces of the digitizer mechanism and eliminate the need for a bezel design.
Another drawback is that, because of the amount of space between the touch surface of the touch screen and display screen, there is a parallax effect. Simply stated, the parallax effect is a type of visual spatial distortion such that the actual point of contact on the touch screen does not correspond to the intended target area of the display screen. This is analogous to a stick being immersed in water, such that the stick takes on a bent or distorted appearance.
Also, the amount of light that comes from the display screen through the touch screen to be viewed by the user is only about 80% of the available light. In a reflective display, that amount is further reduced to about 64%. This reduces the overall contrast, clarity, and quality of the display as seen by the user.
Furthermore, the support structure and outer protective film are typically made of plastic. When subjected to normal use, the support structure and outer protective film can take on a scratched appearance which most users find unappealing. A protective cover may be used to protect the outside of the device, but this is inconvenient in that the device must be removed from the cover to use it and still does not protect the outer protective film while the device is being used.